After booting this system several times, I found the stroage space of mounted read-only file system is descreased. The descrease is not happend quickly. But, for a period of 4~5 days, the space would be descreased about 0.7~1MB. I used the 'df' command to observe this.

Because I had mounted the root file system as read only, why is the storage space descreased? Does any one could show any idea with me? Thanks a lot.